The present invention relates to a power transmission shaft used in apparatuses such as automobiles and industrial machines to transmit torque, and more particularly to a power transmission shaft used in a constant velocity joint.
The present invention also relates to a power transmission shaft that transmits torque via splines or serration.
A power transmission shaft, for example the drive shaft of an automobile, is usually made of carbon steel that is subjected to heat treatment in order to harden the surface thereof, thereby ensuring a predetermined level of strength. Recently, as the automobiles tend to have increasing output power and the vehicle weight increases in order to meet higher safety requirements, the drive shaft is required to have increasingly higher strength. On the other hand, the drive shaft is required to be lighter in weight in order to improve the fuel saving efficiency, which also imposes a pressing need to increase the strength of the drive shaft.
In order to increase the load bearing capacity of a shaft, it is common to increase the strength of the material by increasing the carbon content of the material or other means. With this approach, strength can be increased in smooth-surfaced portions although notched portions such as serration tend to be subjected to quenching crack or other abnormality, leading to lower strength, contrary to the intended purpose. A torsion test was conducted on a serration shaft, and breakage mode was analyzed. The analysis showed that the material is subject to shear fracture when the carbon content is low, but the principal stress becomes dominant as the carbon content increases and the material breaks due to the principal stress (ratio of inter-granular rupture increases). This result also shows the necessity for further inter-granular enhancement of the steel structure. Increasing the carbon content also leads to deterioration of machinability such as forging and cutting.
Accordingly, a first object of the present invention is to further increase the strength and decrease the weight of the power transmission shaft used in a constant velocity joint, without compromising the machinability.
Power transmission shafts for transmitting torque are used in many mechanical components in such apparatuses as automobile and industrial machinery. Among power transmission shafts, spline shafts and serration shafts that transmit high torque, in particular, are manufactured by applying surface hardening treatment such as carburizing, induction hardening or nitriding and/or heat treatment such as thermal refining to medium-carbon steel or low alloy steel to increase the strength of the shaft, while taking into consideration the ease of plastic processing, machinability and cost.
Recently, as the concern about the global environmental issues increases, stricter regulations on the automobile emissions and improved fuel saving efficiency are being called for, and accordingly measures to reduce the weight of the automobile have been taken in order to meet these requirements. Spline shafts and serration shafts are widely used for the drive shaft and propeller shaft of automobiles for coupling with constant velocity joints, and reduction of the weight of the spline shafts and serration shafts makes a great contribution to the weight reduction of an automobile. Thus there is a strong requirement for increasing the strength of these shafts both in terms of static strength and fatigue strength.
Hence a second object of the present invention is to increase the static strength and fatigue strength of a power transmission shaft having torque transmitting teeth such as spline or serration.